There is a general desire to save energy. One field of such energy saving is lighting in buildings, particularly office buildings but also residential buildings. Work is being done to develop highly efficient light sources, which consume less energy and still produce the same amount of light. However, an important energy saving can also be achieved if lights are automatically switched off if they are not needed; in this respect, a light may be considered as being not needed if the area illuminated by this light is not occupied by a person (in practice, more refined definitions can be used). Thus, there is a need for an occupancy sensor.
For being able to switch a lamp, there must be a communication link between the occupancy sensor and the lamp. It is desirable that such link is wireless. This will save on installation costs, and will make it much easier to install occupancy sensors in the case of an already existing lighting infrastructure in already existing buildings. Also, the absence of wires will be aesthetically much more acceptable.
Unavoidably, a sensor device would require power. Power can be supplied from mains, but this requires power lines. Therefore, power is preferably provided by a battery, but in that case the sensor device must have low power consumption in order to have a long life time. In this respect, it would be preferred if the sensor device would be capable of energy harvesting, particularly if the sensor device would be provided with a solar cell, i.e. a cell capable of converting light energy to electric energy. It would even be more preferred if the sensor device would be capable of RF energy harvesting.
Further, from an esthetical point of view, people would not like to have a bulky sensor device mounted at their ceiling or walls. Most desirably, the sensor device should be practically invisible.